Mechanical and chemical-mechanical planarizing processes (collectively "CMP") are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays, and many other microelectronic substrate assemblies. CMP processes generally remove material from a substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate assembly.
FIG. 1 schematically illustrates a rotary CMP machine 10 for planarizing a microelectronic substrate assembly 12. The rotary machine 10 has a platen 20, a wafer carrier assembly 30 above the platen 20, and a polishing pad 40 between the platen 20 and the carrier assembly 30. The carrier assembly 30 generally includes a head 32 to pick up, hold and release the substrate assembly 12 at the appropriate stages of the planarizing process. The carrier assembly 30 can also include a backing pad 34 to support the back side of the substrate assembly 12. The head 32 may be a weighted, free-floating unit, or the carrier assembly 30 can further include an actuator 36 attached to the head 32 to impart axial and/or rotational motion (indicated by arrows C and D, respectively).
The polishing pad 40 can be a non-abrasive polymeric pad (e.g., polyurethane), or it may be a fixed-abrasive polishing pad in which abrasive particles are fixedly dispersed in a resin or another type of suspension medium. A planarizing fluid 44 covers the polishing pad 40 during planarization of the substrate assembly 12. The planarizing fluid 44 may be a conventional CMP slurry with abrasive particles that etch and/or oxidize the surface of the substrate assembly 12, or the planarizing fluid 44 may be a "clean" non-abrasive planarizing solution without abrasive particles. In most CMP applications, abrasive slurries with abrasive particles are used on non-abrasive polishing pads, and non-abrasive cleaning solutions without abrasive particles are used on fixed-abrasive polishing pads.
To planarize the substrate assembly 12 with the CMP machine 10, the carrier assembly 30 presses the substrate assembly 12 face-downward against a planarizing surface 42 of the polishing pad 40. At least one of the platen 20 or the head 32 moves relative to the other to move the substrate assembly 12 across the planarizing surface 42 in the presence of the planarizing solution 44. As the face of the substrate assembly 12 moves across the planarizing surface 42, the polishing pad 40 and/or the planarizing solution 44 continually remove material from the face of the substrate assembly 12.
CMP processes should consistently and accurately produce a uniform, planar surface on substrate assemblies to enable circuit and device patterns to be formed with photolithography techniques. As the density of integrated circuits increases, it is often necessary to accurately focus the critical dimensions of the photo-patterns to within a tolerance of approximately 0.1 .mu.m. Focusing photo-patterns to such small tolerances, however, is difficult when the planarized surfaces of substrate assemblies are not uniformly planar. Thus, to be effective, CMP processes should create highly uniform, planar surfaces on substrate assemblies.
One manufacturing concern of CMP processing is that the surface of the substrate assembly may not be uniformly planar because the rate at which material is removed from the substrate assembly (the "polishing rate") may vary from one area to another. The polishing rate depends, in part, on the relative linear velocity between the surface of the wafer and the portion of the planarizing surface contacting the wafer. The linear velocity of the planarizing surface of a circular, rotating polishing pad varies across the planarizing surface of the pad in proportion to the radial distance from the center of the pad. Similarly, when the head rotates the wafer, the linear velocity also varies across the front face of the wafer in proportion to the radial distance from the center of the wafer. The variation of linear velocities across the face of the wafer and the planarizing surface of the polishing pad creates a relative velocity gradient in between the wafer and the polishing pad. In general, the relative velocity gradient between the wafer and the pad causes a higher polishing rate at the perimeter of the wafer than at the center of the wafer. Such a variance in the polishing rate produces a center-to-edge profile in which more material is removed from the perimeter of the wafer than the center.
Several devices and concepts have been developed to reduce the center-to-edge planarizing profile across wafers. U.S. Pat. No. 5,020,283 issued to Tuttle, which is herein incorporated by reference, discloses a nonabrasive polishing pad with voids in the surface of the pad. The area of the planarizing surface occupied by the voids increases with increasing radial distance to reduce the contact area between the wafer and the planarizing surface of the polishing pad towards the perimeter of the pad. Thus, at the periphery of the pad where the linear velocity of the pad is high, the voids are intended to reduce the polishing rate of the wafer compared to a planarizing surface without such voids.
U.S. patent application Ser. No. 08/834,524 filed by Hudson, which is herein incorporated by reference, discloses an abrasive polishing pad designed to reduce the center-to-edge planarizing profile across or substrate assembly. In one embodiment disclosed in Hudson, the abrasive polishing pad has a planarizing surface with a first planarizing region and a second planarizing region. The first planarizing region has a first abrasiveness and the second planarizing region has a second abrasiveness different than the first abrasiveness of the first region. Hudson discloses that the abrasiveness of the first and second regions can be controlled by using either different types, sizes or densities of abrasive particles fixedly suspended in a suspension medium. Additionally, this application discloses varying the contact/non-contact bearing surfaces on the pad between the first and second regions. The different abrasivity of the first and second planarizing regions are intended to compensate for variations in the relative velocity across the face of the wafer.
Another polishing pad developed to reduce the center-to-edge planarizing profile across a wafer is disclosed in U.S. Pat. No. 5,435,772 issued to Yu, which is also herein incorporated by reference. Yu discloses a circular polishing pad including a first region closer to the edge of the polishing pad and a second region adjacent to the first region toward the center of the polishing pad. The polishing pad disclosed in Yu is configured so that the second region is thicker or less compressible than the first region. Yu states that having a thicker or less compressible portion at the center of the pad and a thinner portion at the perimeter of the pad produces more uniform polishing results.